Method and apparatus for discrimination of ventricular tachycardia from ventricular fibrillation and for treatment thereof

ABSTRACT

An implantable cardioverter/defibrillator provided with method and apparatus for discrimination between ventricular tachycardia and ventricular fibrillation. The device is provided with two pairs of electrodes, each pair of electrodes coupled to processing circuitry for identifying a predetermined fiducal point in the electrical signal associated with a ventricular depolarization. The cumulative beat to beat variability of the intervals separating the two identified fiducal points, over a series of detected depolarizations is analyzed. The result of this analysis is used to distinguish between ventricular tachycardia and ventricular fibrillation.

BACKGROUND OF THE INVENTION

This invention relates to implantable stimulators generally and moreparticularly to implantable cardioverters and defibrillators.

Early automatic tachycardia detection systems for automatic implantablecardioverter/defibrillators relied upon the presence or absence ofelectrical and mechanical heart activity (such as intramyocardialpressure, blood pressure, impedance, stroke volume or heart movement)and or the rate of the electrocardiogram to detect hemodynamicallycompromising ventricular tachycardia or fibrillation. For example, the1961 publication by Dr. Fred Zacouto, Paris, France, entitled,"Traitement D'Urgence des Differents Types de Syncopes Cardiaques duSyndrome de Morgangni-Adams-Stokes" (National Library of Medicine,Bethesda, Md.) describes an automatic pacemaker and defibrillatorresponsive to the presence or absence of the patient's blood pressure inconjunction with the rate of the patient's electrocardiogram to diagnoseand automatically treat brady and tachyarrhythmias.

Later detection algorithms proposed by Satinsky, "Heart MonitorAutomatically Activates Defibrillator", Medical Tribune, 9, No. 91:3,Nov. 11, 1968, and Shuder et al "Experimental Ventricular Defibrillationwith an Automatic and Completely Implanted System", TransactionsAmerican Society for Artificial Internal Organs, 16:207, 1970,automatically detected and triggered defibrillation when the amplitudeof the R-wave of the electrocardiogram fell below a predeterminedthreshold over a predetermined period of time. The initial systemproposed by Mirowski et al in U.S. Pat. No. Re 27,757, which similarlyrelied upon the decrease in the amplitude of a pulsatile rightventricular pressure signal below a threshold over a predeterminedperiod of time, was abandoned by Mirowski et al in favor of the rateand/or probability density function morphology discrimination asdescribed in Mower et al, "Automatic ImplantableCardioverter-Defibrillator Structural Characteristics", PACE, Vol. 7,November-December 1984, Part II, pp. 1331-1334.

More recently, others have suggested the use of high rate plusacceleration of rate or "onset" (U.S. Pat. No. 4,384,585) with sustainedhigh rate and rate stability (U.S. Pat. No. 4,523,595). As stated in thearticle "Automatic Tachycardia Recognition", by R. Arzbaecher et al,PACE, May-June 1984, pp. 541-547, anti-tachycardia pacemakers that wereundergoing clinical studies prior to the publication of that articledetected tachycardia by sensing a high rate in the chamber to be paced.The specific criteria to be met before attempting tachyarrhythmiatermination by pacing involved a comparison of the detected heart rateto a preset threshold, such as 150 beats per minute (400 millisecondcycle length) for a preselected number of beats. As stated above, otherresearchers had suggested the rate of change of rate or suddenness ofonset, rate stability and sustained high rate as additional criteria todistinguish among various types of tachyarrhythmias.

Very generally, the systems that depend upon the aforementioned ratecriteria are capable of discriminating tachycardia in greater or lesserdegree from normal heart rate but can have difficulty discriminatinghigh rate ventricular tachycardias from ventricular fibrillation. Inpractical applications, a common approach has been to specify discreterate zones for ventricular fibrillation and ventricular tachycardia,each defined by minimum rates or minimum R--R intervals. However, insome patients, ventricular tachycardia and ventricular fibrillation mayhave similar rates that make it difficult to distinguish ventricularfibrillation from high rate ventricular tachycardia and supraventriculartachycardias.

Ventricular fibrillation is characterized by chaotic electrical activitywhich presents highly variable depolarization wavefronts which arepropagated in directions which differ from those seen during normalsinus rhythm. Ventricular tachycardias may result from reentryconduction through diseased tissue, which results in depolarizationwavefronts, also typically propagated in directions which differ fromthose seen during normal sinus rhythm. Detection of the occurrence ofdepolarization wavefronts having directions of propagation which differfrom those seen in normal sinus rhythm has been used in various ways indevices intended to detect the presence of ventricular tachycardia orfibrillation.

For example, U.S. Pat. Nos. 3,937,266, 4,088,140 and 4,354,497 describesystems intended to distinguish abnormal ventricular depolarizationwavefronts from depolarization wavefronts which originate in the HISbundle purkinje fiber system. These devices employ a multitude of spacedelectrodes coupled to sense amplifiers and attempt to use the relativearrival times of the wavefronts at the various electrodes to detect theoccurrence of abnormal conduction.

U.S. Pat. No. 4,754,753 presents a method and apparatus for sensing theprobable onset of ventricular fibrillation or pathologictachyarrhythmias by observing the direction of the depolarizationwavefront to predict the onset of harmful ventricular tachyarrhythmias.Detection is accomplished through the use of a multitude of spatiallyoriented electrodes situated on a pacing lead to provide a vectorrepresentation of the direction of propagation of depolarizationwavefronts.

Others, such as the inventors of U.S. Pat. No. 4,712,554, have proposeddistinguishing between sinus and nonsinus atrial depolarizations bydetermining the sequence of atrial activation through the use of bipolaror quadrapolar electrodes placed high in the right atrium. U.S. Pat. No4,577,634 employs quadrapolar atrial and ventricular electrodes fordistinguishing retrograde P-wave conduction from normal sinuspropagation to avoid pacemaker mediated tachycardia. In a further U.S.Pat. No. 4,790,317, it is proposed to recognize ventricular tachycardiaand ventricular fibrillation by comparison of pulse sequences which areobtained when sensing from at least one position on each ventricularepicardial surface. A change in the sequence of activations and in thetiming of signals sensed at the two sensor positions is detected andused to indicate either ventricular tachycardia or ventricularfibrillation.

It has also been proposed in the article entitled "Measurement ofDifference in Timing and Sequence Between Two Ventricular Electrodes asa Means of Tachycardia Differentiation", by Mercando et al, appearing inPACE, Vol. 9, pp. 1069-1078, November-December, 1986, Part II, that theuse of two ventricular sensing electrodes to determine electricalactivation sequence in the expectation that the sequence could provide amethod for differentiation of normal from abnormal rhythms byimplantable antitachycardia devices. Simultaneous recordings from twoventricular sites were obtained during implantation of several devicesor programmed electrical stimulation studies. Recordings were made ofnormal sinus rhythm, ventricular tachycardia, and during prematureventricular contractions. The time intervals between the intrinsicdeflections of the two electrograms derived from the ventricularelectrodes were measured in a number of the patients and the mean andrange values were derived. The authors concluded that the measured meanvalues of the time intervals over a series of beats could be employed inindividual patients to differentiate between normal and abnormalcomplexes. However, while the authors concluded that it would befeasible to detect differences in sequence timing using two ventricularelectrodes in order to distinguish normal sinus beats from ectopicbeats, the disclosed range of mean time intervals shows considerableoverlap.

Yet another proposal for distinguishing between various types oftachyarrhythmia and ventricular fibrillation is disclosed in U.S. Pat.No. 4,799,493 issued to DuFault. In the device disclosed in this patent,the Widrow-Hoff algorithm is utilized for estimation of a transferfunction as a means of discriminating between tachyarrhythmias. Thetransfer function, once determined generates a replica (estimate) of thesignal from a first electrode pair, based on the signal from a secondelectrode pair. The signal from the first electrode pair can besubtracted from the derived replica (estimate) signal to produce a nullsignal, in the presence of stable rhythm. Filters specifically tuned toproduce null signals in the presence of sinus tachycardia or ventriculartachycardia are disclosed, as well as adaptive filters whichautomatically converge in the presence of stable rhythm. Theautomatically adapting filters are disclosed as capable ofdistinguishing between ventricular fibrillation and tachycardias, inthat the LMS algorithms will not allow convergence in the presence offibrillation. This technique is also described in the article "Dual LeadFibrillation Detection for Implantable Defibrillators Via LMS Algorithm"by DuFault et al., published in Computers and Cardiology 1986, IEEEComputer Society Press, pp. 163-166.

SUMMARY OF THE INVENTION

In the context of an automatic implantable device for treatingbradyarrhythmias, tachyarrhythmias and fibrillation, the presentinvention comprises a method and apparatus for reliable discriminationof ventricular fibrillation from high rate monomorphic ventriculartachycardias. The ventricular tachycardia/ventricular fibrillationdiscriminator of the present invention preferably employs two electrodepairs. Each electrode pair is coupled to detection circuitry foridentifying the points in time at which the sensed electrical signalsresulting from the passage of a depolarization wavefront meet certainpredetermined criteria, hereafter referred to as the first and second"fiducial points". The cumulative variability of the time intervalsseparating the occurrence of the first and second fiducial points over aseries of beats is used to distinguish fibrillation from high rateventricular tachycardia.

The electrode pairs may include a common electrode between the twopairs, or may comprise four separate electrodes. The criteria foridentifying the first and second fiducial points may be the same or maydiffer. Identification of the time of occurrence of a first definedfiducial point in the sensed signal from one of the electrode pairs maybe used to define a time window during which the device attempts toidentify a second fiducial point in the sensed signal from the otherelectrode pair. The time interval δ separating the two fiducial pointsassociated with a single detected depolarization wavefront is measuredand stored. The cumulative variability of the value of δ over a seriesof detected depolarization wavefronts in conjunction with detection of ahigh ventricular rate is used to distinguish ventricular fibrillationfrom high rate tachycardia.

The tachycardia/fibrillation discriminator is intended to be used inconjunction with an implantable pacemaker/cardioverter/defibrillatorwhich provides differing therapies for detected ventricular tachycardiasand detected ventricular fibrillation. For example, in response todetection of a tachycardia, the device may provide burst pacing,overdrive pacing or some other antitachycardia pacing regimen.Alternatively, it may provide a low to high energy cardioversion pulse.Typically, in response to detection of fibrillation, the device willprovide a defibrillation pulse at an amplitude significantly higher thana cardioversion pulse.

It is believed that the invention is optimally embodied in a devicewhich is capable of differentiating between low rate tachycardia, highrate tachycardia and fibrillation, and which provides three increasinglyaggressive therapy sets for these three classes of arrhythmias.Detection of low rate tachycardias may be accomplished using any of thenumerous detection methodologies known to the art, as applied todetected heart rates exceeding a lower tachycardia detection rate. Thetachycardia/fibrillation discriminator of the present invention in sucha device will typically be dependant on detection of a heart ratesubstantially in excess of the lower tachycardia detection rate. In suchdevices, the discriminator will serve primarily to distinguish betweenhigh rate tachycardia and fibrillation.

The method and apparatus of the present invention may be convenientlyrealized by providing a first pair of endocardial, myocardial orepicardial electrodes spaced apart from one another in or on theventricles of the heart and a second pair of electrodes which may, forexample, include one of the electrodes of the first pair and a largesurface defibrillation electrode or a remote, indifferent electrode suchas the metal housing of the implantable cardioverter/defibrillator.Alternatively, the second electrode pair might be two large surfacedefibrillation electrodes. Sense amplifiers are provided for eachelectrode pair.

A ventricular electrode pair may be used for detecting the near field,bipolar electrogram and the first fiducial point defined by thecircuitry associated with this electrode pair may correspond totraditional R-wave detection criteria known to the art. The outputsignals from the R-wave detector define the time of occurrence of thefirst fiducial point and also may be used for measuring the duration ofthe intervals separating ventricular depolarizations (R--R intervals) todetermine whether the heart rate is sufficiently rapid to activate thetachycardia/fibrillation discrimination function.

In the event that the detected heart rate is sufficiently rapid, thesignals from the second electrode pair may be analyzed by the detectioncircuitry associated therewith to identify the time of occurrence of thesecond fiducial point. The time interval δ_(i) separating the twofiducal points associated with an individual depolarization wavefront isthen be determined. The beat to beat variation (δ_(i) -δ_(i-1)) of themeasured intervals δ separating the first and second fiducial points ismeasured and the cumulative variability of the values of δ over a seriesof detected depolarization wavefronts is compared to a cumulativevariability threshold to detect fibrillation.

The measurement of cumulative variability may be accomplished bysummation of the beat to beat differences (δi-δ_(i-1)), with detectionof fibrillation occurring when the sum exceeds a cumulative variabilitythreshold. The calculated individual values of (δ_(i) -δ_(i-1)) mayinstead be compared to a threshold value and the number of valuesexceeding the threshold may be counted, with detection of fibrillationoccurring when the count exceeds a cumulative variability threshold.Other measures of cumulative variability may also be employed.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and still further objects, features and advantages of thepresent invention will become apparent from the following detaileddescription of a presently preferred embodiment, taken in conjunctionwith the accompanying drawings, and, in which:

FIG. 1 is a representation of the heart, and an implanted electrodelead, illustrating the theory underlying the present invention.

FIG. 2 illustrates a tranvenous/subcutaneous electrode systemappropriate for use with a pacemaker/cardioverter/defibrillatorembodying the present invention.

FIG. 3 illustrates a myocardial/epicardial electrode system appropriatefor use with a pacemaker/cardioverter/defibrillator embodying thepresent invention.

FIG. 4 is an illustration of the identification of the first and secondfiducial points in sensed signals from two differing pairs ofelectrodes, and of the calculation of the interval δ, separating thefirst and second fiducial points for a single depolarization.

FIG. 5 is a graph illustrating the measurement of the interval δseparating the first and second fiducial points, taken over an extendedseries of detected R-waves or depolarizations during sinus rhythm,ventricular fibrillation, and ventricular tachycardia.

FIG. 6 is a graph illustrating the measurement of variability in themeasured intervals δ separating the first and second fiducial points, asperformed by the tachycardia/fibrillation discriminator of the presentinvention, operating over extended series of detected R-waves in each ofthe three rhythms illustrated in FIG. 5.

FIG. 7 is a schematic block diagram illustrating the structure of oneembodiment of an implantable pacemaker/cardioverter/defibrillator inwhich the present invention may be embodied.

FIGS. 8 and 9 are a functional flow chart illustrating the method ofdiscrimination between ventricular tachycardia and ventricularfibrillation provided by the present invention, and illustrating theoperation of the tachycardia/fibrillation discriminator of the presentinvention as embodied in a microprocessor based device as illustrated inFIG. 7.

FIG. 10 is a block schematic diagram illustrating an alternativeembodiment of an implantable pacemaker/cardioverter/defibrillator inwhich the present invention may be embodied, illustrating a modificationto the circuitry of FIG. 7.

FIG. 11 illustrates an additional alternative embodiment of apacemaker/cardioverter/defibrillator in which the present invention maybe practiced, and also illustrates a modification to the circuitry ofFIG. 7.

FIG. 12 is a functional flow chart illustrating an alternative method ofdiscrimination between ventricular tachycardia and ventricularfibrillation provided by the present invention, and illustrating theoperation of the tachycardia/fibrillation discriminator of the presentinvention as embodied in a microprocessor based device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a cutaway view of the heart, illustrating a ventriculardefibrillation lead carrying a bipolar electrode pair located at theright ventricular apex. The bipolar electrode pair includes a tipelectrode 10, which takes the form of a helical electrode screwed intothe right ventricular myocardium and a ring electrode 12. The lead alsoincludes an elongated coiled defibrillation electrode 14. Theillustrated lead corresponds generally to the leads described in allowedU.S. patent application Ser. No. 07/284,955 by Bardy for an EndocardialDefibrillation Electrode system, but other defibrillation leads may alsobe employed.

In conjunction with FIG. 1, it should be understood that one of the twoelectrode pairs used to sense the first and second fiducial pointsdiscussed above may include ring electrode 12 and tip electrode 10. Theother electrode pair may include ring electrode 12 and a secondelectrode, typically one of the defibrillation electrodes included inthe lead system implanted with the pacemaker/cardioverter/defibrillator.

The general path of propagation of three successive depolarizationwavefronts associated with a sinus rhythm is illustrated by the arrowslabeled "S1, S2, S3". The wavefronts proceed down the septum of theheart, and then expand outward and upward around the right and leftventricles. This pathway of propagation also is present in the case of asupraventricular tachycardia such as a nodal tachycardia or a sinustachycardia, and is consistent from beat to beat.

An example of the general path of propagation of three successivedepolarization wavefronts associated with a monomorphic ventriculartachycardia is illustrated by the arrows labeled "T1, T2, T3". In thecase of monomorphic ventricular tachycardia, the direction ofpropagation, with respect to any particular electrode pair may be thesame or different from that of normal sinus rhythm. However, thedirection of propagation will be approximately the same from beat tobeat.

Also illustrated are examples of the directions of propagation of threesuccessive depolarization wavefronts associated with ventricularfibrillation, illustrated by the arrows labeled "F1, F2, F3". Thehallmark of ventricular fibrillation is the chaotic variation in thespread of the activation wavefront from depolarization to depolarizationas opposed to constancy of wavefront propagation from depolarization todepolarization as seen in supra-ventricular or monomorphic ventriculartachycardia. As illustrated, the direction of wavefront propagation pastelectrodes 10 and 12 varies substantially from one wavefront to thenext.

It is this beat to beat variability in the direction of wavefrontpropagation during fibrillation that allows the discriminator of thepresent invention to distinguish ventricular fibrillation fromventricular tachycardia, whether the tachycardia takes the form of aventricular tachycardia, a supraventricular tachycardia, or a sinustachycardia. In the case illustrated in FIG. 1, the first electrodepair, including electrodes 10 and 12, will sense a different electricalsignal from the second electrode pair, including electrode 12 and asubcutaneous electrode or defibrillation electrode. With varyingdirection of waveform propagation, the relative timing of the twofiducial points derived from the electrode pairs will vary.

While the same detection criteria for the first and second fiducialpoints may be employed, it is believed that it may be advantageous touse differing fiducial point detection criteria for the detectioncircuitry associated with the two electrode pairs. The use of twodiffering fiducial point detection criteria is believed likely toincrease the measured variability of the interval δ between the twofiducial points in the case of ventricular fibrillation withoutcorrespondingly increasing the variability of the time interval δbetween the two fiducial points in the case of a ventriculartachycardia. However, use of either the same or different criteria forthe two fiducal points is believed workable.

FIG. 2 illustrates an implantable pacemaker/cardioverter/defibrillator100 and its associated lead system, as implanted in and adjacent to theheart. As illustrated, the lead system comprises a coronary sinus lead110, a right ventricular lead 120, and a subcutaneous lead 130. Thecoronary sinus lead is provided with an elongated electrode located inthe coronary sinus and great vein region at 112, extending around theheart until approximately the point at which the great vein turnsdownward toward the apex of the heart. The right ventricular lead 120,corresponds to the lead illustrated in FIG. 1, and includes an elongateddefibrillation electrode 122, a ring electrode 124, and helicalelectrode 126, which is screwed into the tissue of the right ventricleat the right ventricular apex. Leads 110 and 120 may correspond to theleads disclosed in allowed U.S. patent Ser. No. 07/284,955 by Bardy foran "Endocardial Defibrillation Electrode System", filed Dec. 15, 1988and incorporated herein by reference in its entirety. A subcutaneouslead 130 is also illustrated, implanted subcutaneously in the leftchest. Lead 130 includes a large surface electrode pad 132, carryingelongated electrode coils 136, 138 and 140. Electrode 132 may correspondto the electrode illustrated in allowed U.S. patent application Ser. No.07/376,730, by Lindemans et al. for a Medical Electrical Lead, filedJul. 7, 1989 and incorporated herein by reference in its entirety.

In conjunction with the present invention, the lead system illustratedprovides numerous electrode pairs which may be employed to practice theinvention. For example, the first electrode pair may comprise ringelectrode 124 and tip electrode 126, with the second electrode paircomprising ring electrode 124 and subcutaneous defibrillation electrode132. Alternatively, the second pair of electrodes could comprisedefibrillation electrode 112 in conjunction with the subcutaneouselectrode 132, or in conjunction with defibrillation electrode 122.

The second electrode pair may instead comprise small surface areaelectrodes (not illustrated) provided on the lead bodies of the coronarysinus lead 110 and/or the ventricular lead 120. For example, anadditional electrode or electrode pair could be mounted to the coronarysinus lead or to the ventricular lead such that the electrode orelectrode pair would be located high in the ventricle or in the superiorvena cava when implanted.

FIG. 3 illustrates an epicardial and myocardial electrode system for usein conjunction with an implantable pacemaker/cardioverter/defibrillator.In this case, two unipolar myocardial electrodes 200 and 202 are locatedon the left ventricle of the heart. These electrodes may correspond tothose illustrated in U.S. Pat. No. 3,737,579, issued to Bolduc on Jun.5, 1973, and incorporated herein by reference in its entirety. Alsoillustrated are three large surface electrodes 204, 206 and 208, spacedaround the ventricles of the heart. These electrodes may correspond tothe electrodes disclosed in U.S. Pat. No. 4,817,634, issued to Hollemanet al. on Apr. 4, 1989, also incorporated herein by reference in itsentirety.

In the context of the present invention, electrodes 200 and 202 mayconstitute the first electrode pair and the second electrode pair mayinclude either of electrodes 200 and 202 in conjunction with one of thelarge surface defibrillation electrodes 204, 206, 208 or may comprisetwo of the defibrillation electrodes.

Alternatively, the first electrode pair might comprise electrode 200 inconjunction either with one of the large surface electrodes 204, 206,208 or in conjunction with an electrode located on the housing of theimplantable device and the second electrode pair might compriseelectrode 202, also paired with one of the large surface area electrodes204, 206, 208 or with a remote indifferent electrode located on thehousing of the implantable pacemaker/cardioverter/defibrillator.

As a practical matter, in the systems as illustrated in FIGS. 2 and 3, apair of small surface area electrodes corresponding generally toelectrodes 124 and 126 in FIG. 2 or to electrodes 200 and 202 in FIG. 3will generally be used for delivery of cardiac pacing pulses and forsensing the occurrence of R-waves in order to reset the timing of thecardiac pacing function and for most purposes associated withtachyarrhythmia recognition. The invention may conveniently be practicedby employing electrodes such as these, in conjunction with an R-wavedetector of a known type to provide a signal indicative of theoccurrence of the first fiducial point as discussed above.

FIG. 4 is a set of human EGM tracings illustrating the detection of thefirst and second fiducial points associated with a detected R-wave andthe measurement of the time interval δ separating them. As illustrated,the top ECG tracing is taken between an electrode pair located in theright ventricle, comprising a tip and ring electrode generallycorresponding to electrodes 124 and 126 illustrated in FIG. 2. The lowertracing is taken between the proximal one of the bipolar pair in theright ventricle, corresponding generally to electrode 124, and a remote,subcutaneous electrode.

As illustrated, the fiducial point identified by the processingcircuitry coupled to the first electrode pair, in the upper tracing,corresponds to the output of an R-wave detection circuit employing abandpass filter followed by a detector having an automatically adjustingthreshold level. The occurrence of the first fiducial point, illustratedby the symbol " ", occurs when the band pass filtered signal from theventricular electrode pair exceeds the detection threshold.

The fiducial point defined by the processing circuitry associated withthe second electrode pair is the detected point of maximum slope of thebandpass filtered signal from the second electrode pair, as illustratedby the symbol " ". The interval δ separating the first and secondfiducial points is also illustrated along with a time window extendingplus or minus 100 milliseconds from the detection of the first fiducialpoint " ", during which detection of the second fiducial point " " isattempted.

As defined in the present application, the time differential δ betweenthe occurrence of the first and second fiducial points is determined bynoting the time of occurrence of the first fiducial point andsubtracting it from the measured time of occurrence of the secondfiducial point. The resulting time interval δ may therefore have apositive or negative value.

FIG. 5 shows a sequence of measured values for δ, as taken during rapidsinus rhythm, ventricular fibrillation and ventricular tachycardia atvarious rates. As illustrated, over a series of detected R-waves, thevalues of δ in the presence of confirmed ventricular fibrillation varyover a substantially greater range than during sinus or othersupraventricular rhythm or during ventricular tachycardia. Testing bythe inventors has shown that the relationship illustrated holds true forventricular tachycardia and ventricular fibrillation even at similarrates (e.g.>250 bpm).

FIG. 6 illustrates the operation of the tachycardia/fibrillationdiscriminator on the data illustrated in FIG. 5. In FIG. 6, theregularity of the value of δ is assessed by means of determining itscumulative beat-to-beat variability over a sequence of eight precedingdetected R-waves using a method hereafter referred to as the "summation"method. The chart begins with the eighth detected R-wave, representingthe first point at which variability has been calculated. Each graphedpoint represents the cumulative variability over the preceding eightvalues of δ. Cumulative variability of the value of δ may be calculatedaccording to the equation: ##EQU1## The cumulative variability ascalculated above is the sum of the absolute values of the beat to beatdifferences between a series of successively measured values of δ. Thesum is compared to a predetermined cumulative variability threshold andif it exceeds the threshold, fibrillation is detected. For example, ifthe cumulative variability over eight measured values of δ exceeds 200ms, fibrillation may be detected.

An alternative method of calculation of cumulative variability for aseries of measured values of δ is hereafter referred to as the "rankordered difference" method. For a series of L measured values of δ, eachmeasured value of δ is compared with M previous values of δ, e.g. (δ_(i)-δi-1), (δ_(i) -δ_(i-2)), (δ_(i) -δ_(i-3)), if M=3. Of these calculateddifferences, the greatest absolute difference (V_(i)) is selected as ameasure of beat to beat variability associated with δ_(i). The L valuesof V_(i) are compared to a predetermined threshold t, and a count C ofthe number of values of V_(i) greater than the threshold is made. Ifmore than K of L values of V_(i) are greater than the threshold, i.e.C>K, fibrillation is detected. An exemplary set of parameters forimplementation of this method of cumulative variability calculationcould be L=8, M=3, t=30 ms and K=2. The inventors of the presentapplication have found that the rank ordered difference method ofcumulative variability assessment provides a high sensitivity to theoccurrence of ventricular fibrillation while maintaining a highspecificity for ventricular tachycardias.

Other calculations of the cumulative variability of the measured valuesof δ, such as the standard deviation of the measured values of δ over aseries of beats, the standard error of the mean measured value of δ overa series of beats or the range of measured values of δ over a series ofbeats might also be used.

As illustrated in FIGS. 5 and 6, the cumulative variability over asequence of detected depolarizations is almost always substantiallyhigher for ventricular fibrillation than for either sinus rhythm or forventricular tachycardia. This has been found to be true using bothmethods of cumulative variability calculation discussed above, but therank ordered difference method is believed to be somewhat superior inavoiding inappropriate detection of fibrillation.

Greater separation of the detected levels of beat to beat variabilityassociated with fibrillation and tachycardia can be accomplished bymeasuring the cumulative variability of the values of δ over a longersequence of detected R-waves. However, this will come at the expense ofextending the period of time the discriminator requires in order tooperate. As a practical matter, it is believed the in most cases,measuring the variability of the values of δ over a sequence of eight totwelve measurements of δ will be adequate to discriminate ventricularfibrillation from ventricular tachycardia or from sinus orsupraventricular tachycardias. For purposes of practical implementation,the number of measurements of δ employed by the discriminator may bemade a programmable parameter, allowing the physician to optimize theoperation of the discriminator to fit the specific needs of individualpatients.

As implemented, the VT/VF discrimination function may be appliedemploying all detected depolarizations in a series of sequential R--Rintervals. Alternatively, the discrimination function may employ onlysome of the detected depolarizations. As discussed below, thediscrimination function is preferably designed such that it provides ameasurement of δ only for individual depolarizations which define theendpoint of R'R intervals sufficiently short to meet the rate criteriafor fast tachycardia or fibrillation detection.

FIG. 7 is a functional schematic diagram of an implantablepacemaker/cardioverter/defibrillator in which the present invention mayusefully be practiced. This diagram should be taken as exemplary of thetype of device in which the invention may be embodied, and not aslimiting, as it is believed that the invention may usefully be practicedin a wide variety of device implementations, including devices havingfunctional organization similar to any of the implantablepacemaker/defibrillator/cardioverters presently being implanted forclinical evaluation in the United States. The invention is also believedpracticable in conjunction with implantablepacemaker/cardioverters/defibrillators as disclosed in prior U.S. Pat.No. 4,548,209, issued to Wielders,et al on Oct. 22, 1985, U.S. Pat. No.4,693,253, issued to Adams et al on Sep. 15, 1987, U.S. Pat. No.4,830,006, issued to Haluska et al on May 6, 1989 and U S. Pat. No.4,949,730, issued to Pless et al on Aug. 21, 1990, all of which areincorporated herein by reference in their entireties.

The device is illustrated as being provided with six electrodes, 500,502, 504, 506, 508 and 510. Electrodes 500 and 502 may be a pair ofelectrodes located in the ventricle, for example, corresponding toelectrodes 124 and 126 in FIG. 2. Electrode 504 may correspond to aremote, indifferent electrode located on the housing of the implantablepacemaker/cardioverter/defibrillator.

Electrodes 506, 508 and 510 may correspond to the large surface areadefibrillation electrodes located on the ventricular, coronary sinus andsubcutaneous leads illustrated in FIG. 2 or to the epicardial electrodes204,206 and 208 of FIG. 3.

Electrodes 500 and 502 are shown as hard-wired to the R-wave detectorcircuit, comprising bandpass filter circuit 514, auto threshold circuit516 for providing an adjustable sensing threshold as a function of themeasured R-wave amplitude and comparator 518. A signal is generated onR-out line 564 whenever the signal sensed between electrodes 500 and 502exceeds the present sensing threshold defined by auto threshold circuit516. As illustrated, the gain on the band pass amplifier 514 is alsoadjustable by means of a signal from the pacer timing and controlcircuitry 520 on GAIN ADJ line 566.

The operation of this R-wave detection circuitry may correspond to thatdisclosed in commonly assigned, copending U.S. patent application Ser.No. 07/612,760, by Keimel, et al., filed Nov. 15, for an Apparatus forMonitoring Electrical Physiologic Signals, incorporated herein byreference in its entirety. However, alternative R-wave detectioncircuitry such as that illustrated in U.S. Pat. No. 4,819,643, issued toMenken on Apr. 11 ,1989 and U.S. Pat. No. 4,880,004, issued to Baker etal on Nov. 14, 1989, both incorporated herein by reference in theirentireties, may also usefully be employed to practice the presentinvention.

The threshold adjustment circuit 516 sets a threshold corresponding to apredetermined percentage of the amplitude of a sensed R-wave, whichthreshold decays to a minimum threshold level over a period of less thanthree seconds thereafter, similar to the automatic sensing thresholdcircuitry illustrated in the article "Reliable R-Wave Detection fromAmbulatory Subjects", by Thakor et al, published in Biomedical ScienceInstrumentation, Vol. 4, pp 67-72, 1978, incorporated herein byreference in its entirety.

In the context of the present invention, it is preferable that thethreshold level not be adjusted in response to paced R-waves, butinstead should continue to approach the minimum threshold levelfollowing paced R-waves to enhance sensing of low level spontaneousR-waves associated with tachyarrhythmias. The time constant of thethreshold circuit is also preferably sufficiently short so that minimumsensing threshold may be reached within 1-3 seconds following adjustmentof the sensing threshold equal to 70-80% of the amplitude of a detectedspontaneous R-wave. The invention may also be practiced in conjunctionwith more traditional R-wave sensors of the type comprising a band passamplifier and a comparator circuit to determine when the bandpassedsignal exceeds a predetermined, fixed sensing threshold.

Switch matrix 512 is used to select which of the available electrodesmake up the second electrode pair for use in conjunction with thepresent invention. The second electrode pair may comprise electrode 502or 500 in conjunction with electrode 504, 506, 508 or 510, or maycomprise other combinations of the illustrated electrodes, includingcombinations of the large surface defibrillation electrodes 506, 508,510. Selection of which two electrodes are employed as the secondelectrode pair in conjunction with the tachycardia/fibrillationdiscrimination function is controlled by the microprocessor 524 viadata/address bus 540. Signals from the selected electrodes are passedthrough bandpass amplifier 534 and into multiplexer 532, where they areconverted to multibit digital signals by A/D converter 530, for storagein random access memory 526 under control of direct memory addresscircuit 528. Microprocessor 524 analyzes the digitized ECG signal storedin random access memory 526 to identify the second fiducial point.

For example, as illustrated in FIG. 4, the microprocessor 524 mayanalyze the ECG stored in an interval extending from minus 100milliseconds previous to the occurrence of an R-wave detect signal online 564, until 100 milliseconds following the occurrence of the R-wavedetect signal in order to identify the second fiducial point.

In the present case, the second fiducal point is the point of maximumslope. However, other fiducial points may also be employed, such as thepoint of maximum amplitude, the detected point of initiation of thedetected depolarization signal or the detected point of termination ofthe depolarization signal. Alternatively, the second fiducal point maybe determined in the same way as the first fiducal point and may be thepoint of detection of the depolarization signal as indicated by a secondR-wave detector coupled to the second electrode pair.

Microprocessor 524 measures the time intervals δ separating the firstand second fiducial points, and stores the measured intervals in randomaccess memory 526. Microprocessor 524 also calculates the cumulativebeat to beat variability of the values of δ over a precedingpredetermined number of measured values of δ, according to either of themethod disclosed above, and determines whether the cumulativevariability is indicative of fibrillation or ventricular tachycardia.

The remainder of the circuitry is dedicated to the provision of cardiacpacing, cardioversion and defibrillation therapies. The pacertiming/control circuitry 520 includes programmable digital counterswhich control the basic time intervals associated with VVI mode cardiacpacing, including the pacing escape intervals, the refractory periodsduring which sensed R-waves are ineffective to restart timing of theescape intervals and the pulse width of the pacing pulses. The durationsof these intervals are determined by microprocessor 524, and arecommunicated to the pacing circuitry 520 via address/data bus 540. Pacertiming/control circuitry also determines the amplitude of the cardiacpacing pulses and the gain of bandpass amplifier, under control ofmicroprocessor 524.

During VVI mode pacing, the escape interval counter within pacertiming/control circuitry 520 is reset upon sensing of an R-wave asindicated by a signal on line 564, and on timeout triggers generation ofa pacing pulse by pacer output circuitry 522, which is coupled toelectrodes 500 and 502. The escape interval counter is also reset ongeneration of a pacing pulse, and thereby controls the basic timing ofcardiac pacing functions, including antitachycardia pacing. The durationof the interval defined by the escape interval timer is determined bymicroprocessor 524, via data/address bus 540. The value of the countpresent in the escape interval counter when reset by sensed R-waves maybe used to measure the duration of R--R intervals, to detect thepresence of tachycardia and to determine whether the minimum ratecriteria are met for activation of the tachycardia/defibrillationdiscrimination function.

Microprocessor 524 operates as an interrupt driven device, and respondsto interrupts from pacer timing/control circuitry 520 corresponding tothe occurrence of sensed R-waves and corresponding to the generation ofcardiac pacing pulses. These interrupts are provided via data/addressbus 540. Any necessary mathematical calculations to be performed bymicroprocessor 524 and any updating of the values or intervalscontrolled by pacer timing/control circuitry 520 take place followingsuch interrupts.

In the event that a tachyarrhythmia is detected, and anantitachyarrhythmia pacing regimen is desired, appropriate timingintervals for controlling generation of antitachycardia pacing therapiesare loaded from microprocessor 524 into the pacer timing and controlcircuitry 520, to control the operation of the escape interval counterand to define refractory periods during which detection of an R-wave bythe R-wave detection circuitry is ineffective to restart the escapeinterval counter. Similarly, in the event that generation of acardioversion or defibrillation pulse is required, microprocessor 524employs the counters to in timing and control circuitry 520 to controltiming of such cardioversion and defibrillation pulses, as well astiming of associated refractory periods during which sensed R-waves areineffective to reset the timing circuitry.

In response to the detection of fibrillation or a tachycardia requiringa cardioversion pulse, microprocessor 524 activatescardioversion/defibrillation control circuitry 554, which initiatescharging of the high voltage capacitors 556, 558, 560 and 562 viacharging circuit 550, under control of high voltage charging line 552.The voltage on the high voltage capacitors is monitored via VCAP line538, which is passed through multiplexer 532, and, in response toreaching a predetermined value set by microprocessor 524, results ingeneration of a logic signal on CAP FULL line 542, terminating charging.Thereafter, delivery of the timing of the defibrillation orcardioversion pulse is controlled by pacer timing/control circuitry 520.One embodiment of an appropriate system for delivery and synchronizationof cardioversion and defibrillation pulses, and controlling the timingfunctions related to them is disclosed in more detail in copending,commonly assigned U.S. patent application Ser. No. 07/612,761, byKeimel, for an Apparatus for Detecting and Treating a Tachyarrhythmia,filed Nov. 15, 1990 and incorporated herein by reference in itsentirety. However, any known cardioversion or defibrillation pulsegeneration circuitry is believed usable in conjunction with the presentinvention. For example, circuitry controlling the timing and generationof cardioversion and defibrillation pulses as disclosed in U.S. Pat. No.4,384,585, issued to Zipes on May 24,1983, in U.S. Pat. No. 4,949,719issued to Pless et al, cited above, and in U.S. Pat. No. 4,375,817,issued to Engle et al, all incorporated herein by reference in theirentireties may also be employed. Similarly,known circuitry forcontrolling the timing and generation of antitachycardia pacing pulsesas described in U.S. Pat. No. 4,577,633, issued to Berkovits et al onMar. 25, 1986, U.S. Pat. No. 4,880,005, issued to Pless et al on Nov.14, 1989, U.S. Pat. No. 7,726,380, issued to Vollmann et al on Feb. 23,1988 and U.S. Pat. No. 4,587,970, issued to Holley et al On May 13,1986, all of which are incorporated herein by reference in theirentireties may also be used.

In the present invention, selection of the particular electrodeconfiguration for delivery of the cardioversion or defibrillation pulsesis controlled via output circuit 548, under control ofcardioversion/defibrillation control circuitry 554 via control bus 546.Output circuit 548 determines which of the high voltage electrodes 506,508 and 510 will be employed in delivering the defibrillation orcardioversion pulse regimen, and may also be used to specify amultielectrode, simultaneous pulse regimen or a multielectrodesequential pulse regimen. Monophasic or biphasic pulses may begenerated. One example of circuitry which may be used to perform thisfunction is set forth in commonly assigned copending patent applicationSer. No. 07/612,758, filed by Keimel, for an Apparatus for DeliveringSingle and Multiple Cardioversion and Defibrillation Pulses, filed Nov.14, 1990, incorporated herein by reference in its entirety. However,output control circuitry as disclosed in U.S. Pat. No. 4,953,551, issuedto Mehra et al on Sep. 4, 1990 or U.S. Pat. No. 4,800,883, issued toWinstrom on Jan. 31, 1989 both incorporated herein by reference in theirentireties, may also be used in the context of the present invention.Alternatively single monophasic pulse regimens employing only a singleelectrode pair according to any of the above cited references whichdisclose implantable cardioverters or defibrillators may also be used.

As discussed above, switch matrix 512 selects which of the variouselectrodes are coupled to band pass amplifier 34. Amplifier 34 may be abroad band pass amplifier, having a band pass extending forapproximately 0.5 to 200 hertz. The filtered EGM signal from amplifier534 is passed through multiplexer 532, and digitized in A-D convertercircuitry 530. The digitized EKG data is stored in random access memory526 under control of direct memory address circuitry 528. Preferably, aportion of random access memory 526 is configured as a looping or buffermemory which stores at least the preceding several seconds of the ECGsignal.

The occurrence of an R-wave detect signal on line 564 is communicated tomicroprocessor 524 via data/address bus 540, and microprocessor 524notes the time of its occurrence. If the tachycardia/fibrillationdiscrimination function is activated, microprocessor 524 waits 100milliseconds following the occurrence of the R-wave detect signal, andthereafter transfers the most recent 200 milliseconds of digitized EGMstored in the looping or buffer memory portion of the random accessmemory circuit 526 to a second memory location, where the contents maybe digitally analyzed to determine the point of maximum slope or otherfiducial point. The transferred 200 milliseconds of stored ECGcorresponds to a time window extending 100 milliseconds on either sideof the R-wave detect signal. For purposes of the present invention, asampling rate of 256 Hz should be sufficient, although somewhat lower orsubstantially higher sampling rates may be used, depending on the amountof data storage capacity in RAM 526 and on the processing speed ofmicroprocessor 524.

In order to determine the point of maximum slope, the microprocessor mayemploy any of a number of known signal analysis techniques, such asthose disclosed in U.S. Pat. No. 4,505,276 issued to Markowitz et al onMar. 19, 1985, incorporated herein by reference in its entirety.Alternatively, Microprocessor 524 may simply compare each stored sampleof the detected R-wave with the preceding sample or with severalpreceding samples to determine the point of maximum slope. A minimumsignal threshold for detection of a point of maximum slope may also beprovided as a prerequisite to identification of a point of maximum slopein order to prevent inappropriate detection due to noise signals.

In the event that no point of maximum slope is identified within the 200milliseconds of stored ECG, random access memory 524 optionally mayenter an arbitrary value For example plus or minus 50 to 100milliseconds from the R-wave detect signal may be entered as the secondfiducal point, with the value alternating between positive and negativewith each successive failure to detect a point of maximum slope. Thefailure to detect a point of maximum slope is most likely to ariseduring fibrillation, and the repeated recording of such alternatepositive and negative values for δ will essentially assure that thecumulative variability measured by the tachycardia/fibrillationdiscriminator will result in a detection of fibrillation, as isappropriate.

Assuming that a point of maximum slope can be identified, the timeinterval separating this second fiducial point from the R-wave detectsignal associated therewith is stored as the value of δ for thatdepolarization wavefront. If the second fiducial point occurs prior tothe R-wave detect signal, the value of δ is negative. If the secondfiducial point occurs after the R-wave detection, δ has a positivevalue. In this fashion, the discriminator is also sensitive to changesin directionality of propagation of the wavefront which do not greatlyaffect the absolute time interval separating the first and secondfiducial points (e.g., a 180 degree reversal of the direction).

After the desired number of values for δ are recorded, and in responseto the detection of a heart rate of sufficient rapidity to indicate theoccurrence of either ventricular tachycardia or fibrillation, themicroprocessor calculates the cumulative variability in the values forδ, according to either method described above. The calculated cumulativevariability is compared to a predetermined threshold value todistinguish tachycardia from fibrillation.

In some embodiments of the invention, it may also be desirable to varythe value of the cumulative variability threshold as a function of theaverage value of the preceding series of R--R intervals. For example, inthe event that the preceding series of R-waves have an average rate of200 beats per minute, a cumulative variability threshold of 100milliseconds (T>100 ms) using the summation method or of 3 values ofV_(i) greater than 30 ms (C>2) using the rank ordered difference method,may be appropriate. However, in the presence of an average rate of 250beats per minute, a cumulative variability threshold of 50 millisecondsusing the summation method or C>2 using the rank ordered differencemethod may be appropriate, to reflect the increased likelihood that suchextremely rapid rates are indicative of fibrillation. Using the rankordered difference method, it may also in some cases desirable todecrease the value of "t" to which V_(i) is compared as the detectedheart rate increases or to vary the number M of values of δ employed tocalculate V_(i).

Similarly, in some embodiments of the invention it may be desirable todefine an upper rate for the operation of the tachycardia/fibrillationdiscriminator, such that heart rates in excess of that upper rate, willalways be detected as fibrillation, regardless of the variability of thestored values of δ. This could also be accomplished by means of anadjustable cumulative variability threshold, gradually reduced from amaximum cumulative variability threshold. For example, using thesummation method, a 100 millisecond cumulative variability threshold atthe minimum rate required for activation of the discriminator functionmay gradually decrease to zero as the average sensed rate heart rateincreases.

FIGS. 8 and 9 are flow charts representing the operation of the deviceillustrated in FIG. 7, in conjunction with the tachycardia/fibrillationdiscrimination function. FIGS. 8 and 9 are intended to functionallyrepresent that portion of the software employed by microprocessor 524(FIG. 7) which implements the tachycardia/fibrillation discriminationfunction. This portion of the software is executed in response to aninterrupt indicating the sensing of a ventricular depolarization at 600.In response to this interrupt, the value of the preceding R--R interval,corresponding to the current time on the escape interval counter inpacer timing/control circuitry 520 may be stored at 602 and used as ameasurement of the R--R interval for tachyarrhythmia detectionfunctions. In addition, the time of detection (DT) of the sensedventricular depolarization, as indicated by means of a real time clockwithin microprocessor 524 is also stored at 602 and serves as the firstfiducal point associated with the detected depolarization.

At 604, the microprocessor updates counters which hold informationregarding the R--R intervals previously sensed. The counters areincremented on the occurrence of a measured R--R interval falling withinan associated rate range. These rate ranges may be defined by theprogramming stored in the RAM 526.

The first range defines a minimum R--R interval used for fibrillationdetection, referred to as "FDI". The associated VF count may reflect thenumber of preceding sequential R--R intervals which are less than FDI,but preferably indicates how many of a first predetermined number of thepreceding R--R intervals were less than FDI.

The second rate range includes R--R intervals less than a fasttachycardia interval "FTDI", and the associated FVT count may indicateeither the number of preceding sequential R--R intervals which are lessthan FTDI and greater than FDI or may indicate how many of a secondpredetermined number of preceding R--R intervals were less than TDIF andgreater than FDI.

The third rate range includes R--R intervals less than a lowertachycardia interval "TDI", and the associated VT count may indicateeither the number of preceding sequential R--R intervals which are lessthan TDI and greater than FTDI or how many of a third predeterminednumber of preceding R--R intervals were less than TDI and greater thanFTDI.

Alternatively, there may be some overlap of the rate zones, such that anR--R interval falling within the overlap zone is counted toward both theFVT and VF counts, or an R--R interval may be counted in all rate zoneshaving defined maximum intervals greater than the measured R--Rinterval.

These counts, along with other stored information reflective of theprevious series of R--R intervals such as information regarding therapidity of onset of the detected short R--R intervals, the stability ofthe detected R--R intervals, the duration of continued detection ofshort R--R intervals, the average R--R interval duration and informationderived from analysis of stored ECG segments are used to determinewhether tachyarrhythmias are present and to distinguish betweendifferent types of tachyarrhythmias. Such detection algorithms forrecognizing tachycardias are described in the above cited U.S. Pat. No.4,726,380, issued to Vollmann, U.S. Pat. No. 4,880,005, issued to Plesset al and U.S. Pat. No. 4,830,006, issued to Haluska et al, incorporatedby reference in their entireties herein. An additional set oftachycardia recognition methodologies is disclosed in the article "Onsetand Stability for Ventricular Tachyarrhythmia Detection in anImplantable Pacer-Cardioverter-Defibrillator" by Olson et al., publishedin Computers in Cardiology, Oct. 7-10, 1986, IEEE Computer SocietyPress, pages 167-170, also incorporated by reference in its entiretyherein. However, other criteria may also be measured and employed inconjunction with the present invention.

For purposes of the present example, the counts may be used to signaldetection of an associated arrhythmia (ventricular fibrillation, fastventricular tachycardia or lower rate ventricular tachycardia) when theyindividually reach a predetermined value, referred to herein as "NID's"(number of intervals required for detection). Each rate zone may haveits own defined NID. Alternatively, detection of an arrhythmia may bebased on the sum of the counts of one or more of the counts reaching apredetermined value (NID) and the particular rate range assigned to thedetected arrhythmia may depend upon the relative values of the variouscounts (e.g. FVT and VF counts) attributable to the most recent seriesof R--R intervals counted toward the NID.

For purposes of the present invention, the particular details ofimplementation of the rate /R--R interval based VF/VT detectionmethodologies are not of primary importance. However, it is required theVF/VT rate detection methodologies employed by the device allowidentification and detection of rhythms in the rate range in whichdistinction between ventricular tachycardia and ventricular fibrillationhas traditionally been difficult, i.e. rates in excess of 200 beats perminute. It is also important that the discriminator function beinitiated far enough in advance of the point at which a tachycardia canbe detected to allow for prior measurement of the required number ofvalues of δ.

If the summation method is used, the discrimination function should beinitiated and measurement of the values of δ begun on or before the timeat which either the VF count or the FVT count equals its respective NID,minus "n", where "n" is the number of measured values of δ employed tocalculate cumulative variability. If the ranked ordered differencemethod is used, the measurement of the values of δ should be initiatedon or before the time at which either the VF or FVT count equals itsrespective NID, minus (L+M), where "L" is the number of measurements ofV_(i) employed to calculate cumulative variability and "M" is the numberof previously stored values of δ compared to the current value of δ todetermine the value of V_(i). In either case, the same result could alsobe accomplished by initiating the measurement of δ in response to the VFcount, the FVT count or the sum of the VF and FVT counts reaching apredetermined value substantially less than their respective NID's.

At 604, the VT, FVT and VF counts are updated as appropriate in responseto the measured R--R interval. At 606, the preceding R--R interval ischecked to determine whether it falls within either the fibrillation orfast ventricular tachycardia rate classes. If the R--R interval fallsinto one of these classes, The discrimination function may be initiated.Otherwise, the heart rhythm will be analyzed to determine whether thedetection criteria for a lower rate tachycardia have been met at 610.

At 608, the VF and VFT counts are analyzed to determine whether thepreceding series of R--R intervals meets the criterion for initiation ofthe VT/VF discrimination function. If the VF count and/or the FVT countmeet the discriminator initiation criterion at 608, the microprocessorinitiates the measurement and collection of the intervals δ separatingthe first and second fiducial points. If not, the microprocessor 524checks whether the measured R--R interval is less than TDI at 610. Ifthe R--R interval is less than TDI, the 200 ms of stored EGM associatedwith the most recent R-wave detect may be analyzed at 612 for waveformcharacteristics indicative of ventricular tachycardia. For example, thewidth of the QRS complex may be measured, the area under the QRS complexmay be measured, or other known types of waveform analysis may beundertaken.

The microprocessor then checks to determine whether the criteria fordetection of a lower rate tachycardia have been met at 614. For example,on the VT count reaching its predetermined NID value and/or the analyzedstored ECG segments meeting predetermined requirements, tachycardia maybe detected. Detection of low rate tachycardia, illustrated functionallyat 614, may correspond to any tachycardia detection algorithm known tothe art. For example, presence of tachycardia may be confirmed by meansof a measurement of average rate, sustained rate, rapid onset, ratestability, or a number of other factors known to the art as discussed inthe above cited patents issued to Pless et al and Haluska et al and inthe Olson et al article. However, one of the advantages of the presentinvention is that it is believed practicable in conjunction withvirtually any prior art tachycardia detection algorithm.

If tachycardia is recognized at 614, the microprocessor 526 initiatesthe scheduled ventricular tachycardia treatment regimen at 616. Inmodern implantable pacemaker/cardioverter/defibrillators, the particulartherapies are programmed into the device ahead of time by the physician,and a menu of therapies is typically provided. For example, on initialdetection of tachycardia, an antitachy pacing therapy may be selected.On redetection of tachycardia, a more aggressive antitachycardia pacingtherapy may be scheduled. If repeated attempts at antitachycardia pacingtherapies fail, a higher level cardioversion pulse therapy may beselected thereafter. Prior art patents illustrating such pre-set therapymenus of antitachyarrhythmia therapies include the above-cited U.S. Pat.No. 4,830,006, issued to Haluska, et al, U.S. Pat. No. 4,727,380, issuedto Vollmann et al and U.S. Pat. No. 4,587,970, issued to Holley et al.The present invention is believed practicable in conjunction with any ofthe known antitachycardia pacing and cardioversion therapies, and it isbelieved most likely that the invention of the present application willbe practiced in conjunction with a device in which the choice and orderof delivered therapies is programmable by the physician, as in currentimplantable pacemaker/cardioverter/defibrillators.

After delivery of a tachycardia therapy at 616, the counters are resetat 618. At 620, the microprocessor updates the tachyarrhythmia detectionmethodologies. As discussed in the above-cited patents, in some cases itis desirable to have a different standard for redetection of atachyarrhythmia than for initial detection of the tachyarrhythmia.Typically the criteria for redetection will be less stringent than forinitial detection. Similarly, at 622, the microprocessor updates thetherapy schedule, to reflect that the previously scheduled therapy hadbeen delivered. As discussed above, in current implantablepacemaker/cardioverter/defibrillators, this generally results in thedelivery of a more aggressive therapy upon redetection of tachycardia.After updating the tachyarrhythmia related functions, the microprocessorreturns the device to VVI mode bradycardia pacing and awaits the nextinterrupt at 600.

In the event that ventricular tachycardia is not detected at 614, themicroprocessor checks at 623 to determine if a tachyarrhythmia waspreviously detected and is not indicated to have been terminated. If nounterminated tachyarrhythmias are indicated, the microprocessor returnsthe device to the bradycardia pacing. If a tachyarrhythmia was detectedpreviously, the microprocessor checks at 624 to determine whether thepreceding series of R--R intervals, including the most recent, indicatesa return to sinus rhythm or termination of a previously detectedarrhythmia. The criterion of detection of return to sinus rhythm may bea series of a predetermined number of sequential R--R intervals whichare greater than TDI, for example. Tachycardia termination criteria asset forth in the above cited Pless et al, Haluska et al or other priorart termination detection criteria may also be used. Followingtermination detection, the counters, detection methodologies and therapyschedules are all appropriately updated at 618, 620 and 622 and thedevice returns to VVI mode pacing, as discussed above.

In the event that the discriminator activation criterion is met at 608,the microprocessor begins measuring and collecting the values of δ. At626, the value, "i" indicative of the number of the present measurementof δ is incremented, and the time of occurrence of the sensedcontraction or ventricular pacing pulse at 600 (DT) is entered asT_(i1). One hundred milliseconds following the detection of aventricular depolarization at 600, the portion of the random accessmemory serving as the EGM buffer is frozen, and the most recent 200milliseconds of digitized EGM signal is transferred to a separate memorylocation within the random access memory for analysis. Under control ofmicroprocessor 524, the point of maximum slope is identified at 628.Using the digital data processing techniques discussed above, the pointof maximum slew may be identified as either the point of greatest slope,positive or negative, or may be identified as the point of greatestpositive slope or the point of greatest negative slope, as may bedesired. If a point of maximum slew is identified as indicated at 630,the time of maximum slew is set equal to T_(i2) at 632. If no maximumslew point is found, an arbitrary value, for example plus or minus 50milliseconds as discussed above may be recorded for the value of T_(i2),at 630. Assuming that values of T_(i1) and T_(i2) have successfully beenentered, δ_(i) is calculated at 636, and stored.

While the embodiment discussed in conjunction with FIGS. 7-9 alternatelymeasures the maximum slope and the width of a detected signal indicativeof a depolarization, both forms of analysis could be performed with alldetected depolarizations associated with R--R intervals indicative ofany tachyarrhythmia, if sufficient computational speed is available.However, the alternate use of width and slope analysis for low ratetachycardia and VT/VF discrimination, respectively, is believed toprovide a workable approach in devices having computational and memorycapabilities in line with the current and probable next generation ofimplantable pacemaker/cardioverter/defibrillators.

At 638, the microprocessor checks to determine whether the VF count orthe FVT count is greater than or equal to their corresponding NID's,indicating detection of high rate ventricular tachycardia orfibrillation. As discussed above, an alternative criterion for detectionof ventricular fibrillation of fast ventricular tachycardia may employthe sum of the VF and FVT counts. If ventricular fibrillation or fastventricular tachycardia is not detected, the microprocessor returns tothe lower rate tachycardia detection function at 614.

In the event that fast ventricular tachycardia or ventricularfibrillation detection criteria are met at 638, the discriminatorcalculation is performed at 640, measuring the cumulative variabilityover the preceding series of R--R intervals using one of the two methodsdiscussed above. The value of i is reset to zero at 642. At 644, thecalculated cumulative variability is compared with the cumulativevariability threshold using one of the methods discussed above. If thecumulative variability is greater than or equal to the threshold,fibrillation therapy is selected at 646. If the cumulative variabilityis less than the threshold, fast ventricular tachycardia therapy isselected at 648.

Therapies for fast ventricular tachycardia may be of the same generaltypes provided in conjunction with detection of ventricular tachycardiaat 616 (FIG. 8), and may include antitachycardia pacing andcardioversion pulse therapies. However, the therapy menu for fastventricular tachycardia will be more aggressive than the therapy set forslower ventricular tachycardias. For example, fewer or no attempts atantitachy pacing may be undertaken prior to delivery of cardioversionpulses. Higher amplitude cardioversion pulses may be specified.

The references cited above in conjunction with descriptions of prior arttachycardia detection and treatment therapies are applicable here aswell. Again, the focus of the present invention is to distinguishfibrillation from tachycardias. It is believed that in the context ofpracticable devices, the physician will be provided with the ability toselect which of a number of available therapies are provided in responseto the detection of slow or fast tachycardias.

In the event that fibrillation is identified at 646, the typical therapywill be delivery of a high amplitude defibrillation pulse, typically inexcess of 10 joules, and in some cases as much as 35 joules or more. Asin the case of currently available implantablepacemakers/cardioverter/defibrillators, and as discussed in theabove-cited references, it is envisioned that the amplitude of thedefibrillation pulse may be incremented in response to failure of aninitial pulse or pulses to terminate fibrillation.

Following delivery of the defibrillation pulse or tachycardia therapy,the tachyarrhythmia functions are updated at 618, 620 and 622 to reflectthe delivery of the selected therapy. The microprocessor then returns tothe device to VVI pacing and, and awaits the next successive interruptdue to ventricular pacing or the occurrence of a sensed ventriculardepolarization at 600.

FIG. 10 is a block diagram of an alternative embodiment of the presentinvention, substituting somewhat different circuitry for the portion ofthe circuitry illustrated in FIG. 7 which performs thetachycardia/fibrillation discrimination function. In the apparatusillustrated in this figure, both fiducial points are determined by meansof digital signal processing. A switch matrix 710 is provided whichselects which of four electrodes 702, 704, 706 and 708 are coupled toband pass amplifiers 714 and 716. Band pass amplifiers 714 and 716 mayhave the same band pass characteristics, or may have differing band passcharacteristics if desired. The signals from band pass amplifiers 714and 716 are provided to digital multiplexer 718, which alternatelysamples the signal from band pass amplifiers 714 and 716 and applies itto A/D converter 720. The output of A/D converter 720 is stored inrandom access memory 724 under control of direct memory addresscircuitry 726. Preferably, portions of random access memory 724 areconfigured as data buffers, each capable of storing the precedingseveral seconds of digitized EGM signals. The stored EGM signals areanalyzed by microprocessor 722 to detect the relative times ofoccurrence of first and second defined fiducial points, recording thedifference in their occurrence times (δ), and performing thetachycardia/fibrillation detection discrimination function as discussedabove. The defined fiducial points may be the same, or may be different,with regard to the stored digitized ECG signals from buffer amplifiers714 and 716. Similarly, the electrodes coupled between amplifiers 714and 716 by switch matrix 710 may be two separate sets of electrodes, twosets of electrodes with one electrode in common, or may in some cases,employ the same set of electrodes, provided that the selected fiducialpoints are different from one another. Selected fiducial points mayinclude, for example, the identified beginning or end of sensed R waves,the point of maximum amplitude of sensed R waves, the point of maximumslope of sensed R waves, or may be based on other identifiablecharacteristics of the recorded EGM signals.

FIG. 11 shows a second alternative embodiment, in which dedicatedcircuitry is employed to perform the fiducial point detection and themeasurement of the time intervals δ separating the first and secondfiducial points. Similar to the apparatus illustrated in FIG. 10, theapparatus of FIG. 11 is provided with a set of electrodes 802, 804, 806and 808 which are applied to amplifiers 816 and 814 by switch matrix810. Included in this embodiment of the invention are separate,dedicated fiducial point detection circuitry blocks 818 and 820. Onedetection block, for example block 818, may correspond to an R wavedetector as discussed in conjunction with FIG. 7. The second detector820 may, for example, be an analog circuit capable of detecting thepoint of maximum amplitude of the sensed R wave, or may include adifferentiation circuit coupled to a peak detector for detecting themaximum slope of the sensed R wave. In any case, circuits 818 and 820provide output pulses upon occurrence of their respectively detectedfiducial points. The output pulse from circuitry 818 initiates a timewindow of 200 milliseconds determined by timer 822. The signal fromdetection circuitry 820 is delayed by 100 milliseconds by timer 824.

On generation of the detection signal from circuitry 818, timer 826 isreset, and begins timing. Following the earliest of the time out oftimer 822 or timer 824, the clock signal to timer 826 is disabled via ORgate 828. The value stored in timer 826, at this point, will reflect thetiming difference between the detection of the two fiducial points.

The combination of timers 822, 824 and 826 produce a system in which, ifboth fiducial points occur at the same time, the timer 826 will hold acount corresponding to 100 milliseconds. Thus, phase information in theform of the order of occurrence of the two fiducial points is preserved,and can be recovered by the microprocessor by subtracting a valueequivalent to 100 milliseconds from the stored time interval in timer826, to provide either a positive or a negative value of δ.Microprocessor 830 and random access memory 832 correspond to thoseillustrated in FIG. 7, are interconnected with the remainder of thecardioversion, defibrillation and pacing circuitry in a manner analogousto that illustrated in FIG. 7. Alternatively, microprocessor 830 couldbe replaced by full custom digital circuitry or even analog circuitrydedicated specifically to performance of the discriminator function.

FIG. 12 is a generic functional flow chart intended to illustrate thebasic mechanism by which the VF/VT discrimination function of thepresent invention may be added to or employed with a prior artimplantable pacemaker/cardioverter/defibrillator of the type whichprovide differing levels of therapy depending the diagnosedtachyarrhythmia. The flow chart of FIG. 12 is purely a functional flowchart, and may reflect the operation of a microprocessor device or mayequally well reflect the operation of a device fabricated employing fullcustom digital circuitry, or even analog circuitry.

At 900, it is assumed that a ventricular depolarization is sensed or aventricular pacing pulse is delivered, while the device is operating inVVI bradycardia pacing mode. The R--R interval ending with the senseddepolarization or the delivered pacing pulse is stored and measured at910, and the history of recent stored R--R intervals is updated at 812to reflect the most recently measured R--R interval.

At 914, the device reviews the R--R interval history to determinewhether criteria for activating the VT/VF discriminator are met. Asdiscussed above, this is preferably a sequence of R--R intervalsindicating a high likelihood a cardiac rhythm which could be diagnosedas either ventricular fibrillation or fast ventricular tachycardia isunderway. In the event that the discriminator criteria are met at 914,the value of δ_(i) is measured at 916, and the device steps through itshierarchy of detection criteria.

As illustrated, the device is provided with five detection criteriaincluding ventricular fibrillation detection criteria at 918, VT/VFdiscriminator zone criteria at 922, fast ventricular tachycardiacriteria at 928, ventricular tachycardia criteria at 932 and arrhythmiatermination or sinus rhythm detection criteria at 936. The deviceattempts to classify the current cardiac rhythm into one of these fivecategories, based on the R--R interval history. As illustrated, separateVF zone, discrimination zone and FVT zone criteria are envisioned.However, as disclosed in conjunction with the flow charts in FIGS. 8 and9, these three classifications may all be reduced to a singleclassification, if the operative zone of operation of the VT/VFdiscriminator is extended to include any cardiac rhythm meeting eitherVF or FVT detection criteria.

At 918, the R--R interval history is checked to determine whether itunambiguously identifies the occurrence of ventricular fibrillation. Forexample, a rate in excess of 250∝280 beats per minute may be used tounambiguously identify ventricular fibrillation. In the event thatventricular is identified, ventricular fibrillation therapy, typically asingle high energy pulse is delivered at 920.

In the event that a rhythm which might be diagnosed as eitherventricular fibrillation or fast ventricular tachycardia is detected at922, the cumulative variability of the measured values of δ iscalculated, and the cumulative variability is compared to a thresholdvalue at 926. For example, R--R intervals indicative of a cardiac rhythmof greater than 180-200 beats per minute, but less than 250-280 beatsper minute might define the rate discriminator operation zone specifiedat 922. In the event that the cumulative variability is less than thethreshold cumulative variability, a fast tachycardia therapy, such ascardioversion pulses or antitachycardia pacing is delivered at 930. Inthe event that the cumulative variability is greater than the thresholdvariability at 926, ventricular fibrillation therapy is delivered at920.

In the event that fast ventricular tachycardia is unambiguouslyidentified at 928, fast ventricular tachycardia therapy is delivered at930. For example, fast ventricular tachycardia may be identified inresponse to heart rhythms having a rate between 180 and 200 beats perminute. As discussed above, an alternative is to define a VF/VTdiscriminator rate zone which extends through and includes both the fastventricular tachycardia and ventricular fibrillation rate zones,allowing for omission of the functional blocks illustrated at 918 and928.

In the event that the rhythm is diagnosed as a slower tachycardia at932, a less aggressive ventricular tachycardia therapy is delivered at934, for example, a series of attempts at antitachycardia pacing may beprescribed. In the event that tachycardia is not detected, the devicechecks at 936 to determine whether a tachyarrhythmia had previously beendetected and whether the tachyarrhythmia has been terminated, i.e.,whether the patient has returned to a normal sinus rhythm. For example,a sequence of several R--R intervals at less than the rate required todetect ventricular tachycardia may be used to detect termination.Tachycardia detection and treatment functions such as the R--R intervalhistory, the detection criteria for the various rate zones and thetherapy menus may be updated at 938, and the device returns tobradycardia pacing mode at 940, to await the next sensed ventricularcontraction or ventricular pacing pulse at 900. Similarly, afterdelivery of any of the various antitachycardia or defibrillationtherapies, the tachyarrhythmia detection and therapy menus are updatedat 938 to reflect the tachyarrhythmias detected and the therapiesdelivered, and the device returns to bradycardia pacing at 940. In theevent that none of the detection criteria are met, as indicated by afailure to detect sinus rhythm or termination of tachyarrhythmia at 936,the device simply returns to the brady pacing mode, and awaits the nextsubsequent sensed ventricular contraction or ventricular pacing pulse at900.

FIGS. 10 and 11 are included to illustrate the scope of applicability ofthe present invention, and that it can be practiced in conjunction withimplantable pacemaker/cardioverter/defibrillators in which signalprocessing is done in an essentially analog fashion, in essentiallydigital fashion, or any mix thereof. FIGS. 10 and 11 are also intendedto indicate that the scope of the invention should not be construed aslimited by the functional schematic of FIG. 7, which, like FIGS. 10 and11 should be considered illustrative, rather than limiting with regardto the scope of the claims that follow.

FIG. 12 is intended to illustrate the broad applicability of the VT/VFdiscriminator provided by the present invention, and provide generalguidance as to incorporation of the invention intopacemaker/cardioverter/defibrillators of any of the known types and infuture such devices. The broad applicability of the discriminator in thepresent application is believed to be one of its most valuableattributes.

Furthermore, it should be recognized that although the disclosedembodiment deals with fibrillation and tachycardia in the lower chambersor ventricles of the heart, the invention may be usefully practiced inthe context of the upper chambers or atria of the heart, which are alsoprone to tachycardia and fibrillation in some patients. In addition,while the therapies discussed in conjunction with the disclosedembodiment generally relate to delivery of electrical pulses, it shouldbe understood that the invention may be usefully practiced inconjunction with any device adapted to deliver differing therapies fortachycardia and fibrillation, including drug therapies, non-pulsatileelectrical therapies, and any other such therapies as may be implementedin such devices as their development progresses, whether applieddirectly to the heart or systemically.

Similarly, it should be understood that the discriminator of the presentinvention, while particularly adapted for use in or in conjunction withan implantable cardioverter/defibrillator may also in some cases beusefully practiced in conjunction with a nonimplantable device, in adevice which, for example only treats fibrillation or only treatstachycardia, or even in a device adapted primarily for diagnosticpurposes.

In conjunction with above application, we claim:
 1. A cardioverter/defibrillator, comprising: treatment means for delivering a first therapy to a patient's heart to treat tachycardia and a second therapy to said patient's heart to treat fibrillation;first means for sensing electrical signals from said patient's heart indicative of the depolarization of a chamber or chambers of said patient's heart; second means for sensing electrical signals from said patient's heart indicative of the depolarization of said chamber or chambers of said patient's heart; first fiducial point detection means, coupled to said first sensing means for determining the time at which the electrical signal sensed by said first sensing means meets first predetermined criteria and for issuing a first fiducial point signal indicative thereof; second fiducial point detection means, coupled to said first sensing means for determining the time at which the electrical signal sensed by said second sensing means meets second predetermined criteria and for issuing a second fiducial point signal indicative thereof; and tachycardia/fibrillation discriminator means responsive to said first and second fiducial point signals for measuring the time interval separating the times at which said electrical signals associated with a single depolarization of said chamber or chambers of said patient's heart meet said first and second predetermined criteria and for measuring the cumulative variability of said measured time interval over a series of depolarizations of said chamber or chambers of said patient's heart and for selecting between said first and second therapies as a function of said measured variability.
 2. A cardioverter/defibrillator, comprising:treatment means for delivering a first therapy to a patient's heart to treat tachycardia and a second therapy to said patient's heart to treat fibrillation; first means for sensing electrical signals from said patient's heart indicative of the depolarization of a chamber or chambers of said patient's heart, said first sensing means comprising a first electrode pair; second means for sensing electrical signals from said patient's heart indicative of the depolarization of said chamber or chambers of said patient's heart, said second sensing means comprising a second electrode pair having at least one electrode not included in said first electrode pair; first fiducial point detection means, coupled to said first sensing means for determining the time at which the electrical signal sensed by said first sensing means meets first predetermined criteria associated with the depolarization of said chamber or chambers of said patient's heart and for issuing a first fiducial point signal indicative thereof; second fiducial point detection means, coupled to said first sensing means for determining the time at which the electrical signal sensed by said second sensing means meets second predetermined criteria associated with the depolarization of said chamber or chambers of said patient's heart and for issuing a second fiducial point signal indicative thereof; and tachycardia/fibrillation discriminator means responsive to said first and second fiducial point signals for measuring the time interval separating the times at which said electrical signals associated with a single depolarization of said chamber or chambers of said patient's heart meet said first and second predetermined criteria and for measuring the cumulative variability of said measured time interval over a series of depolarizations of said chamber or chambers of said patient's heart and for selecting between said first and second therapies as a function of said measured variability.
 3. A cardioverter/defibrillator, comprising:treatment means for delivering a first therapy to a patient's heart to treat tachycardia and a second therapy to said patient's heart to treat fibrillation; first means for sensing electrical signals from said patient's heart indicative of the depolarization of a chamber or chambers of said patient's heart; second means for sensing electrical signals from said patient's heart indicative of the depolarization of said chamber or chambers of said patient's heart; first fiducial point detection means, coupled to said first sensing means for determining the time at which the electrical signal sensed by said first sensing means meets first predetermined criteria associated with the depolarization of said chamber or chambers of said patient's heart and for issuing a first fiducial point signal indicative thereof; second fiducial point detection means, coupled to said first sensing means for determining the time at which the electrical signal sensed by said second sensing means meets second predetermined criteria associated with the depolarization of said chamber or chambers of said patient's heart, said first criteria differing from said second criteria, and for issuing a second fiducial point signal indicative thereof; and tachycardia/fibrillation discriminator means responsive to said first and second fiducial point signals for measuring the time interval separating the times at which said electrical signals associated with a single depolarization of said chamber or chambers of said patient's heart meet said first and second predetermined criteria and for measuring the cumulative variability of said measured time intervals over a series of depolarizations of said chamber or chambers of said patient's heart and for selecting between said first and second therapies as a function of said measured variability.
 4. A cardioverter/defibrillator according to claim 1 or claim 2 or claim 3 wherein said tachycardia/ fibrillation discriminator means comprises means for comparing individual ones of said measured time intervals with another said measured time interval or intervals to derive an individual measurements of variability associated with said individual measured time intervals and means for determining the cumulative variability of said measured time intervals based upon said individual measurements of variability .
 5. A cardioverter/defibrillator according to claim 4 wherein said means for determining comprises means for summing said individual measurements of variability and means for determining whether the sum exceeds a variability threshold.
 6. A cardioverter/defibrillator according to claim 4 wherein said means for determining comprises means for determining whether said individual measurements of variability exceed a variability threshold and for determining the number of said individual variability measurements which exceed said variability threshold.
 7. A cardioverter/defibrillator according to claim 6 wherein said means for comparing comprises means for comparing individual ones of said measured time intervals with more than one other of said measured time intervals and for selecting the greatest differences between said compared measured time intervals as the said individual variability measurements associated with the said individual ones of said measured time intervals.
 8. A cardioverter/defibrillator, comprising: treatment means for delivering a first therapy to a patient's heart to treat tachycardia and a second therapy to said patient's heart to treat fibrillation;first means for sensing electrical signals from said patient's heart indicative of the depolarization of a chamber or chambers of said patient's heart and for issuing a first fiducial point signal indicative thereof, said first sensing means comprising a first electrode pair; second means for sensing electrical signals from said patient's heart indicative of the depolarization of said chamber or chambers of said patient's heart and for issuing a second fiducial point signal indicative thereof, said second sensing means comprising a second electrode pair including at least one electrode not included in said first electrode pair; tachycardia/fibrillation discriminator means responsive to said first and second fiducal point signals for cumulatively measuring the depolarization to depolarization variability of the direction of depolarization wavefront propagation within said chamber or chambers of said patient's heart over a series of depolarizations of said patient's heart and for selecting between said first and second therapies as a function of said measured variability.
 9. A cardioverter/defibrillator according to claim 8 wherein said criteria for said first fiducal point differs from said criteria for said second fiducal point.
 10. A cardioverter/defibrillator, comprising: treatment means for delivering a first therapy to a patient's heart to treat tachycardia and a second therapy to said patient's heart to treat fibrillation;first means for sensing electrical signals from said patient's heart indicative of the depolarization of a chamber or chambers of said patient's heart and for issuing a first fiducial point signal indicative thereof in response to said signals from said patient's heart meeting first criteria; second means for sensing electrical signals from said patient's heart indicative of the depolarization of said chamber or chambers of said patient's heart and for issuing a second fiducial point signal indicative thereof in response to said signals from said patient's heart meeting second criteria differing from said first criteria; tachycardia/fibrillation discriminator means responsive to said first and second fiducal point signals for cumulatively measuring the depolarization to depolarization variability of the direction of depolarization wavefront propagation within said chamber or chambers of said patient's heart over a series of depolarizations of said patient's heart and for selecting between said first and second therapies as a function of said measured variability.
 11. A cardioverter/defibrillator according to claim 8 or claim 10 or claim 9 wherein said tachycardia/fibrillation discriminator means comprises means responsive to said first and second fiducial point signals for measuring the time interval separating said first and second fiducal point signals.
 12. A cardioverter/defibrillator according to claim 11 wherein said tachycardia/fibrillation discriminator means comprises means for measuring the cumulative variability of said measured time intervals over a series of depolarizations of said chamber or chambers of said patient's heart.
 13. A defibrillator, comprising:treatment means for delivering a therapy to a patient's heart to treat to treat fibrillation; first means for sensing electrical signals from said patient's heart indicative of the depolarization of a chamber or chambers of said patient's heart and for providing a first fiducal point signal indicative thereof, said first sensing means comprising a first electrode pair; second means for sensing electrical signals from said patient's heart indicative of the depolarization of said chamber or chambers of said patient's heart and for providing a second fiducal point signal indicative thereof, said second sensing means comprising a second electrode pair including at least one electrode not included in said first electrode pair; fibrillation detection means responsive to said first and second fiducal point signals for cumulatively measuring the depolarization to depolarization variability of the direction of depolarization wavefront propagation within said chamber or chambers of said patient's heart over a series of depolarizations of said patient's heart and for detecting fibrillation and initiating said therapy as a function of said measured variability.
 14. A defibrillator, comprising:treatment means for delivering a therapy to a patient's heart to treat to treat fibrillation; first means for sensing electrical signals from said patient's heart indicative of the depolarization of a chamber or chambers of said patient's heart and for providing a first fiducal point signal indicative thereof in response to said electrical signals meeting first predetermined criteria; second means for sensing electrical signals from said patient's heart indicative of the depolarization of said chamber or chambers of said patient's heart and for providing a second fiducal point signal indicative thereof in response to said electrical signals meeting second predetermined criteria differing from said first predetermined criteria; fibrillation detection means responsive to said first and second fiducal point signals for cumulatively measuring the depolarization to depolarization variability of the direction of depolarization wavefront propagation within said chamber or chambers of said patient's heart over a series of depolarizations of said patient's heart and for detecting fibrillation and initiating said therapy as a function of said measured variability.
 15. A defibrillator according to claim 14 wherein said criteria for said first fiducal point differs from said criteria for said second fiducal point.
 16. A defibrillator according to claim 13 or claim 14 or claim 15 wherein said tachycardia/fibrillation discriminator means comprises means responsive to said first and second fiducial point signals for measuring the time interval separating said first and second fiducal point signals.
 17. A defibrillator according to claim 16 wherein said tachycardia/fibrillation discriminator means comprises means for measuring the cumulative variability of said measured time intervals over a series of depolarizations of said chamber or chambers of said patient's heart.
 18. A cardioverter, comprising:treatment means for delivering a therapy to a patient's heart to treat to treat tachycardia; first means for sensing electrical signals from said patient's heart indicative of the depolarization of a chamber or chambers of said patient's heart and for providing a first fiducal point signal indicative thereof, said first sensing means comprising a first electrode pair; second means for sensing electrical signals from said patient's heart indicative of the depolarization of a chamber or chambers of said patient's heart and for providing a second fiducal point signal indicative thereof, said second sensing means comprising a second electrode pair including at least one electrode not included in said first electrode pair; tachycardia detection means responsive to said first and second fiducal point signals for cumulatively measuring the depolarization to depolarization variability of the direction of depolarization wavefront propagation within said chamber or chambers of said patient's heart over a series of depolarizations of said patient's heart and for detecting tachycardia and initiating said therapy as a function of said measured variability.
 19. A cardioverter, comprising:treatment means for delivering a therapy to a patient's heart to treat to treat tachycardia; first means for sensing electrical signals from said patient's heart indicative of the depolarization of a chamber or chambers of said patient's heart and for providing a first fiducal point signal indicative thereof in response to said electrical signals meeting first criteria; second means for sensing electrical signals from said patient's heart indicative of the depolarization of a chamber or chambers of said patient's heart and for providing a second fiducal point signal indicative thereof in response to said electrical signals meeting second predetermined criteria differing from said first criteria; tachycardia detection means responsive to said first and second fiducal point signals for cumulatively measuring the depolarization to depolarization variability of the direction of depolarization wavefront propagation within said chamber or chambers of said patient's heart over a series of depolarizations of said patient's heart and for detecting tachycardia and initiating said therapy as a function of said measured variability.
 20. A tachycardia/fibrillation discriminator according to claim 19 wherein said criteria for said first fiducal point differs from said criteria for said second fiducal point.
 21. A cardioverter according to claim 19 wherein said criteria for said first fiducal point differs from said criteria for said second fiducal point.
 22. A cardioverter according to claim 18 or claim 19 or claim 21 wherein said tachycardia/fibrillation discriminator means comprises means responsive to said first and second fiducial point signals for measuring the time interval separating said first and second fiducal point signals.
 23. A cardioverter according to claim 22 wherein said tachycardia/fibrillation discriminator means comprises means for measuring the cumulative variability of said measured time intervals over a series of depolarizations of said chamber or chambers of said patient's heart.
 24. A tachycardia/fibrillation discriminator, comprising:first means for sensing electrical signals from said patient's heart indicative of the depolarization of a chamber or chambers of said patient's heart and for providing a first fiducal point signal indicative thereof, said first sensing means comprising a first electrode pair; second means for sensing electrical signals from said patient's heart indicative of the depolarization of said chamber or chambers of said patient's heart and for providing a second fiducal point signal indicative thereof, said second sensing means comprising a second electrode pair including at least one electrode not included in said first electrode pair; means responsive to said first and second fiducal point signals for measuring the cumulative variability of the direction of depolarization wavefront propagation within said chamber or chambers of said patient's heart over a series of depolarizations of said patient's heart and for providing an output indicative of the detection of tachycardia or fibrillation as a function of said measured variability.
 25. A tachycardia/fibrillation discriminator, comprising:first means for sensing electrical signals from said patient's heart indicative of the depolarization of a chamber or chambers of said patient's heart and for providing a first fiducal point signal indicative thereof in response to said electrical signals meeting first criteria; second means for sensing electrical signals from said patient's heart indicative of the depolarization of said chamber or chambers of said patient's heart and for providing a second fiducal point signal indicative thereof in response to said electrical signals meeting second criteria differing from said first criteria; means responsive to said first and second fiducal point signals for measuring the cumulative variability of the direction of depolarization wavefront propagation within said chamber or chambers of said patient's heart over a series of depolarizations of said patient's heart and for providing an output indicative of the detection of tachycardia or fibrillation as a function of said measured variability.
 26. A tachycardia/fibrillation discriminator according to claim 24 or claim 25 or claim 20 wherein said responsive means comprises means responsive to said first and second fiducial point signals for measuring the time interval separating said first and second fiducal point signals.
 27. A tachycardia/fibrillation discriminator according to claim 26 wherein said responsive means comprises means for measuring the cumulative variability of said measured time intervals over a series of depolarizations of said chamber or chambers of said patient's heart.
 28. A tachycardia/fibrillation discriminator according to claim 27 wherein said responsive means comprises means for comparing individual ones of said measured time intervals with another said measured time interval or intervals to derive individual measurements of variability associated with said individual measured time intervals and means for determining the cumulative variability of said measured time intervals based upon said individual measurements o variability.
 29. A tachycardia/fibrillation discriminator according to claim 28 wherein said means for determining comprises means for summing said individual measurements of variability and means for determining whether the sum exceeds a variability threshold.
 30. A tachycardia/fibrillation discriminator according to claim 28 wherein said means for determining comprises means for determining whether said individual measurements of variability exceed a variability threshold and for determining the number of said individual variability measurements which exceed said variability threshold.
 31. A tachycardia/fibrillation discriminator according to claim 30 wherein said means for comparing comprises means for comparing individual ones of said measured time intervals with more than one other of said measured time intervals and for selecting the greatest differences between said compared measured time intervals as the said individual variability measurements associated with the said individual ones of said measured time intervals. 